The invention relates to antiscattering grids and particularly to antiscattering grids used in X-ray imaging. A radiology imaging apparatus may comprise an X-ray source and an image receiver between which the object, of which it is wished to produce an image, is positioned. The beam emitted by the source passes through the object before reaching the detector. It is partly absorbed by the internal structure of the object so that the intensity of the beam received by the detector is attenuated. The overall attenuation of the beam after having passed through the object is directly related to the absorption distribution in the object.
The image receiver may comprise an optoelectronic detector or enhancing film/screen pair sensitive to the intensity of the radiation. The image generated by the receiver corresponds in principle to the distribution of the overall attenuations of the rays due to having passed through the internal structures of the object.
One part of the radiation emitted by the source is absorbed by the internal structure of the object and the other part is either transmitted (primary or direct radiation) or scattered (secondary or scattered radiation). The presence of scattered radiation results in the contrast of the image obtained being degraded and the signal/noise ratio being reduced.
One solution to this problem comprises interposing one or more “antiscattering” grids between the object to be X-rayed and the image receiver. This grid is usually formed by a series of parallel plates made of a material that absorbs the X-rays. In a grid identified as a “focused” grid (using the terminology defined by the CEI 60627 standard relating to “X-ray imaging diagnostic equipment—characteristics of the antiscattering grids for general use and for mammography”), all the planes of the plates intersect along the same straight line passing through the focal point of the radiation emitted by the source.
A antiscattering grid may comprise a series of oriented parallel plates made of a material which strongly absorbs the X-rays, for example, lead, and are held together between inter-plate members made of a material more transparent to X-rays than the plates, such as, for example, aluminum or cellulose fibers (paper or wood). A disadvantage for such a grid is that the grid is unable to reduce the degree of radiation scattered in a direction parallel to the plane of the plates. Crossed grids having two series of absorbent plates, in which the two plates are positioned perpendicular to each other, reduce this disadvantage. The crossed grids allow two-dimensional filtering to be obtained and thus further reduce the solid angle with which a point on the receiver sees the object.
However, the presence of inter-plate members between the plates reduces the transmission of the direct radiation through the grid. Consequently, the X-ray dose needed to obtain an image of good quality must be increased (particularly in mammography).
There are several structural arrangements for obtaining grids without inter-plate members. One structural arrangement is two-dimensional filtering comprising superposing one-dimensional grids. U.S. Pat. No. 5,307,394 describe an antiscattering device comprising the superposition of several grids. Each grid has parallel absorbent plates separated by openings, the plates being positioned at an angle of between 0 and 89.9° with respect to the central beam perpendicular to the plane of the receiver. The grids have a thickness such that the ratio of the thickness of the grid to the distance between the plates is greater than 1. This arrangement provides filtering of the rays whose angle of incidence is high.
Another structural arrangement comprises producing two-dimensional grids directly in a one-piece support. U.S. Pat. No. 5,389,473 describes a method of manufacturing an antiscattering grid, comprising producing a grid from a glass plate, making openings in the plate by photoetching and chemical etching (for example using hydrofluoric acid). Such a method results in an array of cells separated by partitions being formed. The partitions are then covered with a layer of material that absorbs the X-rays.
WO 94/17533 describes an antiscattering grid comprising a lattice formed in a glass plate, the lattice comprising cells separated by partitions. The partitions are covered with a layer of absorbent material, not only inside the cells but also along their edges corresponding to the upper and lower surfaces of the glass plate.
In the field of mammography, another type of two-dimensional grid has been developed. U.S. Pat. No. 5,606,589 describe an antiscattering grid comprising thin metal sheets micro-etched to form a lattice. The sheets are superposed so as to form a focused grid. The sheets are held stacked together by adhesive bonding.
U.S. Pat. No. 5,814,235 describes a method of manufacturing such a grid. The method comprises forming, by photoetching in a metal foil, a lattice structure comprising cells separated by segments extending in transverse directions. The foils are then individually immersed in a bath of adhesive. The foils are then stacked on top of each other so as to align the segments in order to form partitions between the cells and then clamped together in position. This method makes it possible in particular to obtain strong grids.
U.S. Pat. No. 6,075,840 generalizes this type of grid for applications other than mammography.
The disadvantage with the above methods is that they are based on the chemical etching of layers of materials and consequently result in a significant loss of material, and hence high costs.
Moreover, a disadvantage with the antiscattering grids in general is that they mask part of the image receiver and leave their impression on the image obtained. A solution to this problem comprises slightly shifting the grid during image acquisition. Thus, the image obtained comprises the superposition of images, for which the grid lies in different positions.
In certain cases, it is possible to translate the grid only in a single direction (in particular in mammography, in which the direction of displacement is parallel to the patient's thoracic cage). In this case, the impressions of the segments or of the partitions which are positioned parallel to the direction of displacement of the grid remain on the image obtained. U.S. Pat. No. 5,970,118 describes an antiscattering grid formed in a glass plate in which the partitions of the cells are not parallel to the side of the grid parallel to the direction of displacement of the latter. For example, the grid may comprise cells having a square shape and oriented at 35° with respect to an edge of the grid parallel to the direction of displacement.